Method of preparing biaxially oriented polyethylene 2,6-naphthalate film

ABSTRACT

A biaxially oriented polyethylene 2,6-naphthalate film having superior physical properties in the machine direction such as tensile strength and Young&#39;&#39;s modulus, particularly useful as a base of magnetic recording tape, is prepared by stretching an undrawn film of polyethylene 2,6-naphthalate consisting essentially of ethylene 2,6-naphthalate units and having an intrinsic viscosity of at least 0.35, first in the machine direction to 3.5-5 times the original length at a temperature ranging from 10*C. above the second order transition temperature of said polyethylene 2,6-naphthalate to 170*C., and then in the transverse direction at a draw ratio 50-90 percent of that of the machine direction stretching at a temperature ranging from 3*C. above said second order transition temperature to 160*C. and satisfying the following relationship

United States Patent Tanabe et al. [451 Aug. 8, 1972 [54] METHOD OF PREPARING 'BIAXIALLY ORIENTED POLYETHYLENE 2,6- Primary Examiner-Robert F. White NAPHTHALATE FILM Assistant Examiner-Jeffery R. Thurlow [72] Inventors: Takashi Tanabe, Kanagawa-ken; Ammeysheman and shalloway Hiroshi Aoiki, Tokyo; Hitoshi Mura- TR q vkarni, Fujio Matsumoto, both of ABS CT Sagamihara-shi, Kanagawaken, all A blaxlally oriented polyethylene 2,6-naphthalate film fja a having superior physical properties in the machine direction such as tensile strength and Young's modu- [73] Assign: -P" lus, particularly useful as a base of magnetic recording tape, is prepared by stretching an undrawn film of [22] Flled' June 1970 polyethylene 2,6-naphthalate consisting essentially of [21] Appl. No.: 44,588 ethylene 2,6-naphthalate units and having an intrinsic viscosity of at least 0.35, first in the machine direction to 3.5-5 times the original length at a temperature [30] Forelgn Apphcauon Pnomy Data ranging from l0C. above the second order transition June 13, 1969 Japan ..44/46574 temperature of said polyethylene 2,6-naphthalate to 170C., and then in the transverse direction at a draw [52] US. Cl ..264/289, 260/75 T, 264/235, ratio 50-90 percent of that of the machine direction 264/346 stretching at a temperature ranging from 3C. above [51] Int. Cl. ..B29d 7/22, B29d 7/24, C08g 17/00 Said second order transition temperature to 160C. [58] Field of Search ..264/289, 210, 234, 235, 345, n sati fying he following relationship 264/346; 260/75 T 25x- 770" 1200 T 22x 400" 700, [56] References Cited wherein T is the drawing temperature (C.) when stretching the film in the transverse direction, X is the UNITED STATES PATENTS draw ratio when stretching the film in the transverse 2 968 065 l/l96l Gronholz ..264/289 direction, and n is the refraetive index in the thiekneee 2 975 484 3/1961 Amborski ..260/75 T direction of the film after stretching in the machine direction.

3,161,711 12/1964 Tassler ..264/289 4 Claims, No Drawings METHOD OF PREPARING BIAXIALLY ORIENTED POLYETHYLENE 2,6-NAPH'IHALATE FILM This invention relates to a method of preparing biaxially oriented polyethylene 2,6-naphthalate film. More particularly, the invention relates to a method of preparing biaxially oriented polyethylene 2,6-naphthalate film wherein the undrawn film is stretched at a greater draw ratio in the machine direction than the transverse direction to thereby provide a film which excels especially in its physical properties such, for example, as tenacity, Youngss modulus and dimensional stability in the machine direction.-

Films whose physical properties have been especially improved in the machine direction are used, for example, as the base support of magnetic recording tape as well as other continuous films. Such films have hitherto been made from cellulose acetate or polyethylene terephthalate. However, there is a limit to .the tensile strength and dimensional stability in the machine direction in the case of the cellulose acetate film, and moreover it has the drawback that owing to its small modulus of elasticity a magnetic recording tape below a given thickness cannot be made therefrom. On the other hand, in the case of the polyethylene terephthalate film, an increase in the draw ratio in the machine direction which is considerably greater than that in the transverse direction cannot be made by the customary biaxialstretching technique, i.e., the two-stage stretching method wherein the elongation is effected successively in the machine and transverse directions (in the sequence given) and, as a result, it is impossible to obtain directly by the foregoing biaxial stretching technique a film whose physical properties has been improved in the machine direction to such a degree as is presently being demanded. Accordingly, it has been proposed lately to produce polyethylene terephthalate film with improved physical properties in the machine direction by a three-stage method, i.e., stretching the film successively in the machine, transverse and machine directions or a two-stage biaxial stretching method, i.e., first stretching the film in the transverse direction and then in the machine direction. These films are referred to as tensilized type" film. However,

there are drawbacks in that the tensilized film of polyethylene terephthalate, while demonstrating a satisfactory tensile strength or Youngs modulus in the machine direction, does not always exhibit a sufficiently high thermal dimensional stability. In addition,

since the foregoing successive three-stage stretching method requires a three-stage operation in carrying out the stretching, the operation becomes complicated. On the other hand, in the case of the biaxial stretching method in which the stretching of the film in the transverse direction is first carried out followed by the stretching in the machine direction, there is the drawbackthat the sideways drawing machine (tenter) must be a heavy duty one because the thicker film must be machine direction and consequently, while the Youngs modulus becomes greater in the machine direction, the heat shrinkage in the machine direction also increases concomitantly therewith.

It is therefore an object of the present invention to provide a method of producing directly by the readily carried out biaxial stretching operation, wherein the stretching is first effected in the machine direction and then in the transverse direction, a polyethylene 2,6- naphthalate film which excels in physical properties such as tensile strength and Youngs modulus in the machine direction and possesses a low heat shrinkage and hence has good dimensional stability.

Another object of the invention is to provide a new method of producing a very thin polyethylene 2,6 naphthalate film whose gauge variation is small and whose dimensional stability is high.

One specific object of the invention is to provide a method of producing a film suitable for use as the base support of magnetic recording tape.

Other objects and advantages of the invention will become apparent from the following description wherein the present invention is more fully described.

As a result of research into the preparation of films from various polymers, it has been discovered that polyethylene 2,6-naphtha1ate had a distinctive filmforming property not seen in the other crystalline polyesters and crystalline polyolefins. In the past, when the undrawn film obtained from polyethylene terephthalate or a crystalline polyolefin of isotactic structure was, as previously noted, stretched in the machine'direction at, e.g., a draw ratio of 3-5, or even greater, at a temperature somewhat higher than the second order transition temperature of these polymers, the molecules were oriented in the machine direction and in concomitance therewith some crystallization took place. Hence, when a film which has been stretched in the machine direction is subsequently stretched in the transversedirection at a draw ratio lower than the draw ratio at which it was previously stretched in the machine direction, the film is stretched non-uniformly and numerous stretched portions and unstretched portions are formed, thus making it impossible to make a film having a uniform thickness and good physical properties. Consequently, in preparing a film having uniform transverse profile by the biaxial stretching of undrawn films such as hereinabove mentioned wherein the stretching in the machine direction is first carried out, it was generally the practice heretofore to use a draw ratio in the transverse direction of at least equal to that used in the machine direction and, in addition, to use a slightly higher stretching temperature in the transverse direction than in the case of the machine direction.

According to researche, however, it has been found that the undrawn film of polyethylene 2,6-naphthalate, unlike the foregoing undrawn film of polyethylene terephthalate or isotactic polypropylene, has a distinctive characteristic that, as previously indicated, even when the film is first stretched in the machine direction at a high draw ratio of 3-5 and thereafter stretched in the transverse direction at a lower draw ratio, a film with a good transverse profile is obtained. Furthermore, it was found that when this stretching in the transverse direction is carried out at a'temperature within a specific range with respect to the draw ratio in the transverse direction and the refractive index of the film after having conducted the stretching of the film in the machine direction, and preferably at a temperature not higher than the stretching temperature that was used in carrying out the preceding stretching in the machine direction, a film with very few gauge variations can be formed. Thus, in accordance with the present invention, biaxially oriented films of polyethylene 2,6-naphthalate having especially improved tensile strength and Youngs modulus in the machine direction can be obtained by a simple twostage stretching operation.

A description of the present invention in still further detail is given hereunder.

According to the present invention, a uniform film of polyethylene 2,6-naphthalate which has been biaxially oriented, as previously noted, and moreover has especially, excellent tensile strength and Youngs modulus in the machine direction can be prepared by stretching an undrawn film made of polyethylene 2,6-naphthalate consisting essentially of ethylene 2,6-naphthalate units and having an intrinsic viscosity of at least 0.35,

l. in the machine direction at a draw ratio of 3.5-5 to the original length of the undrawn film at a temperature ranging between C. above the second order transition temperature of the polyethylene 2,6-naphthalate and 170C., and thereafter 2. in the transverse direction at a draw ratio 5090 percent of the draw ratio at which the film was stretched in the machine direction, at a temperature ranging between 3C. above the aforesaid second order transition temperature and 160C, such temperature further satisfying the following relationship wherein T is the stretching temperature (C.) in the transverse direction, X is the draw ratio in the transverse direction, and n is the refractive index in the thickness direction of the film whose stretching in the machine direction has been completed. The method of measuring the refractive index, n, of the foregoing relationship and the intrinsic viscosity above will be described later. The polyethylene 2,6-naphthalate to be used as the base polymer of the undrawn film used in the invention will be satisfactory for use if it is one consisting essentially of ethylene 2,6-naphthalate units. Included are not only polyethylene 2,6-naphthalate but also the modified polymers of ethylene 2,6-naphthalate whose modification has been by means of a small quantity, e.g., not more than 10 mol percent, and preferably not more than 5 mol percent, of a third component.

In general, polyethylene 2,6-naphthalate is synthesized by bonding naphthalene 2,6-dicarboxylic acid or the functional derivatives thereof with ethylene glycol or the functional derivatives thereof under suitable reaction conditions in the presence of a catalyst. One or more suitable third components (modifier) may be added before completion of the polymerization of the polyethylene 2,6-naphthalate to form a copolymeric or mixed polyester. As the foregoing third component, the compounds having a bivalent ester forming functional group such, forexample, as dicarboxylic acids as oxalic, succinic, adipic, phthalic, isophthalic, terephthalic, naphthalene 2,7-dicarboxylic and diphenylether dicarboxylic acids or the lower alkyl esters thereof can be used; or such known compounds as dihydric alcohols such as propylene glycol and trimethylene glycol can be used. Again, the polyethylene 2,6-naphthalate or its modified polymer may be one whose terminal hydroxyl and/or carboxylic group has been capped by a monofunctional compound such, for example, as benzoic acid, benzoylbenzoic acid, benzyloxybenzoic acid and methoxypolyalkylene glycol; or one modified with a trifunctional ester-forming compound within a scope such that the copolymer obtained is essentially linear.

Further, the foregoing polyester may also contain delustrants such as titanium dioxide, stabilizers such as phosphoric and phosphorous acid and the esters thereof, and additives such as finely divided silica and china clay.

The polymer made up of essentially ethylene 2,6- naphthalate units, as hereinbefore described, is referred to as polyethylene 2,6-naphthalate in the present invention and, of these naphthalate polyesters, those whose intrinsic viscosity is 0.35 or more are used. One whose intrinsic viscosity is less than 0.35 is not desirable, since a useful film cannot be obtained. As the polyethylene 2,6-naphthalate to be used in the invention, one having an intrinsic viscosity of 0.50-0.80 is especially to be preferred from the standpoint of the properties of the resulting product as well as from the standpoint of the stretching operation.

The invention method is applied to an undrawn film formed from a naphthalate polyester such as hereinbefore described. In forming the undrawn film from the naphthalate polyester, the conventional methods, for example, the melt-extrusion technique, can be employed.

According to the invention, in first stretching the undrawn film in the machine direction and thereafter in the transverse direction, the stretching in the machine direction is carried out at a draw ratio of 3.5-5 to the original length of the undrawn film, at a temperature ranging between 10C. above the second order transition temperature and C. The preferred draw ratio at which the film is stretched in the machine direction.

is 4-5 to the original length of the undrawn film, and particularly desirable results are obtained when the undrawn film is stretched at a temperature range of l30l50C. and at a draw ratio of 4-5 to its original length.

The reason why the temperature and draw ratio must be controlled as hereinabove indicated is because when the initial drawing temperature is lower than the aforesaid temperature which is 10C. above the second order transition temperature, cold drawing results and only a turbid film of low tenacity and low Youngs modulus can be obtained. On the other hand, when the drawing temperature exceeds 170C, though the resulting film appears to have good transparency, adequate orientation is not imparted because of flow drawing, with the consequence that a film having satisfactory mechanical properties cannot be obtained. Further when the draw ratio in the machine direction is less than 3.5, the tenacity and Youngs modulus desired cannot be obtained and, on the other hand, when this draw ratio exceeds 5, the subsequent operation of stretching in the transverse direction becomes difficult. Thus, for the reasons given above, it is especially convenient that in the present invention the stretching in the machine direction is carried out at a temperature in the range of l30-l50C. and at a draw ratio of 4-5 to the undrawn films original length.

In the present invention, after having stretched the film in the machine direction, the stretching thereof in the transverse direction is carried out at a draw ratio 50-90 percent, and preferably 60 70 percent, of that in the machine direction, at a temperature ranging between 3C. above the second order transition temperature of the polyethylene 2,6-naphthalate and 160C. and moreover a temperature that satisfies the following relationship (the temperature T therein) 25X770n+l200 T 5 22X400n+700 wherein T is the drawing temperature (C.) in the transverse direction, X is the draw ratio at which the film is stretched in the transverse direction, and n is the refractive index in the thickness direction of the film whose stretching in the machine direction has been completed, as hereinbefore described.

The reason why the temperature condition is controlled, as hereinabove indicated, in carrying out the stretching in the transverse direction is because when the stretching is carried out at a temperature lower than 3C. above the second order transition temperature of the polyethylene 2,6-terephthalate used, cold drawing takes place to give rise to the film either becoming turbid or breaking; while, on the other hand, when the temperature exceeds 160C., the film tends to be stretched irregularly and, in addition, with an increase in the tendency of flow drawing taking place it will be impossible to obtain a film having excellent mechanical properties.

For the reason given above, the stretching in the transverse direction according to the present invention is carried out at a temperature ranging between 3C. above the second order transition temperature of the polyethylene 2,6-naphthalate used and 160C., but, as hereinbefore indicated, it is further necessary to carry out this stretching in the transverse direction at a temperature which satisfies the following relationship 25X-770bn+l200 'T .22X-400n+700 wherein T, X and n are as hereinbefore defined.

The reason for this is that the aforesaid temperature "PC. to be used in carrying out the stretching of the film in the transverse direction must be suitably chosen in accordance with the degree of orientation and degree of crystallization that have been set up in the film as a result of its stretching in the machine direction that has been carried out in advance of the stretching in the transverse direction as well as the draw ratio at which the stretching in the transverse direction is to be actually carried out. When this temperature in carrying out the stretching in the transverse direction exceeds (22X400n+700)C., even a slight gauge variation in the starting undrawn film gives rise to marked gauge variation of the drawn film and at times a biaxially oriented film in which stretched and unstretched portions are conjointly present is obtained. On the other hand, when this temperature in carrying out the stretching in the transverse direction is lower than (25X-770n+1200)C., the resulting film either becomes turbid or its breakage takes place. Therefore, the temperature in carrying out the stretching in the transverse direction is conveniently one which is in the range of l20-l40C. and, in addition, satisfies the hereinbefore indicated relationship.

Further, by conducting the stretching in the transverse direction within the hereinabove indicated range and furthermore at a temperature not higher than that in stretching the film in the machine direction the invention makes possible the preparation of a biaxially oriented film having an especially small amount of the gauge variation and hence is an advantage. Further, when the invented two-stage stretching method of stretching the film in the machine direction followed by stretching it in its transverse direction is carried out directly subsequent to the formation of the undrawn film by, e.g., the melt-extrusion method, there is the advantage that by controlling the temperature in this manner in conducting the stretching in the transverse direction the thickness regulating operation during the extrusion operation of the aforesaid undrawn film becomes simplified.

The stretching in the transverse direction is carried out within the temperature range, as hereinabove indicated, and at a draw ratio of 50-90 percent, and preferably 60-70 percent, of the draw ratio at which the film was stretched in the machine direction.

When the draw ratio of the stretching in the transverse direction becomes less than 50 percent of that of the stretching in the machine direction, the tear strength in the machine direction of the resulting film becomes low and film tends to flex (bend) in the transverse direction, while, on the other hand, when the draw ratio becomes greater than percent of that at which it was stretched in the machine direction, the Youngs modulus and tensile strength of the resulting film in the machine direction becomes low, with the consequence that the obtainment of a film excelling in its properties in the machine direction directly by the two-stage stretching method, as intended by the present invention, cannot be achieved. For the foregoing reason, it is of particular advantage to ensure that the stretching in the transverse direction is carried out so that the draw ratio is 60-70 percent of that at which it was stretched in the machine direction.

According to the invention, a film excelling in both the machine and the transverse directions, and especially the machine direction, in such physical properties as, for example, tensile strength, Youngs modulus and dimensional stability can be obtained by conducting the two-stage stretching in the machine and the transverse directions in the sequence given.

Again, in accordance with the invention, when the biaxially oriented film obtained after having carried out the hereinabove indicated two-stage stretching in the machine and the transverse directions is heat-treated further at a temperature in the range of 250C., and preferably l80-230C., the heat shrinkage of said biaxially oriented film becomes small and its thermal stability is improved. The heat treatment can be carried out be exposing the biaxially oriented film to the atmosphere of foregoing temperature, e.g., for about 2 seconds -5 minutes, and preferably about 4-60 seconds. When the heat treatment temperature becomes higher than the range indicated above, the

gauge variation of the film tends to increase and, in addition, there is the drawback that the molecular orien-' tation of the film is disarranged to cause a decline of its Young's modulus. On the other hand, when the temperature of the heat treatment is lower than that indicated, heat treatment effects are not fully demonstrated. in either case of a higher or lower temperature than that indicated, desirable results are unobtainable.

According to the invention, by a simple stretching operation consisting of a two-stage stretching of the undrawnfilm in the machine and the transverse directions in the sequence given followed further by a heat treatment, a polyethylene 2,6-naphthalate film possessing a small heat shrinkage, great thermal stability, and a small amount of gauge variation, as well as the previously noted excellent physical properties can be obtained. in addition, the film obtained in this invention excels in its resistance to hydrolysis.

Hence, the film prepared by the invention process finds wide use for such purpose as, for example, electrical insulating materials such as electric wire covering tape and adhesive tape as well as for various kinds of continuous film.

The measurement values given in the specification and claims and the following examples were values obtained in the following manner. Intrinsic viscosity Intrinsic viscosity is calculated from the equation:

limit 31 c o C wherein C is the concentration in grams of the polymer in 100 ml. solvent and m is the relative viscosity mea sured at 35C. The solvent used herein is a mixture of 6 parts of phenol and 4 parts of orthodichlorobenzene (by weight). Refractive index Measurement was made with a light of wavelength 589 mp. (center of the D lines) at a temperature of 20C. using an Abbe refractometer.

Film gauge variation A film from whose both edges were trimmed a 7-cm strip was continuously measured for its thickness over the entire width of the film by means of a beta-ray thickness measuring instrument. The value of the gauge variation was obtained as follows from the maximum and minimum thickness values.

Gauge variation:

intrinsic viscosity Heat shrinkage=- 10 X 0141) Tensile strength at break, elongation at break, Young's modulus and stress at 5 elongation These values were measured by the following methods in an atmosphere of 23C. and 65 percent relative humidity, using an lnstron tensile testing machine.

Method A.

Shape of specimen: Rectangular (15 cm long,

1 cm wide) Distance between grips at start of test:

Strain rate:

l0 cm l0 cm/min.

Method B.

Shape of specimen: Dumbbell (total length 10 cm,

length of narrow part 4 cm, width of gripping part 2 cm, width of narrow part 1 cm) Distance between grips at start of test: Strain rate:

5 cm 2 cm/min.

On the other hand, the evaluation of the stretchability in the examples was in accordance with the following 5 standard.

2 Average thickness A. The stretching takes place apparently uniformly.

B. Conjoint presence of stretched and unstretched portions are observed in the film and thus nonuniformity is outwardly apparent.

C. Breakage or devitrification takes place during the stretching operation.

Examples l-6 and Controls l-6 An undrawn film 200 microns in thickness and 40 cm in width prepared using polyethylene 2,6-naphthalate of intrinsic viscosity 0.70 was first stretched at a draw ratio of either 3.5, 4.0 or 4.5 in the machine direction at 130C, after which it was stretched at a draw ratio of 3.0 in the transverse direction varying the drawing temperatures, whereupon the relationship between the drawing temperature, stretchability and film properties X such as shown in Table l was obtained.

TABLE 1 Refractive index in thickness Drawing Gauge variation Draw direction a'ter temperature stretchability in transverse ratio in stretching transverse when stretched direction of machine in machine direction in transverse resulting film Experiment N0. direction direction C.) direction (percent) 3. 5 1. 550 C 3. 5 1. 550 A :l;3 3. 5 1. 550 A $5 3. 5 1. 550 A :l:20 4. 0 1. 535 115 C 4. 0 1. 635 125 A $4 4. 0 1. 535 135 A :!;5 4. 0 1. 535 A 5:23 4. 5 1. 525 115 C 4. 5 1. 525 120 A $3 4. 5 1. 525 140 A :l:6 Control 6 4. 5 1. 525 B I235 From the foregoing results, it is apparent that in the ceeding 90 percent of the draw ratios they were case of the films obtained in Examples 1-6 which stretched in the machine direction, the Youngs modusatisfy the conditions of the present invention as to the lus in the machine direction was unsatisfactory. On the temperature used when stretching the films in the other hand, it is seen that in the case of the films of transverse direction, their stretchability was g and 5 Controls and 11 in which the films were stretched in the g g v ria n w small in COhIIaSI, n h Case Of the transverse direction at draw ratios less than 50 perthefilms obtained in Controls 2 and 4 hich d not cent of draw ratios at which they were stretched in the satisfy the conditions of the invention as to the temhi di ti n, th t r strength in the machine Perature used when stl'etehing the films in the transdirection became exceedingly poor. In contrast, the Verse direction the g g Variation of the films films of Examples 7-16 according to the invention mined was excessive t g y could pp y be method excelled in their Youngs modulus in the Stretched uniformly- Hence y lacked usefulness- A5 machine direction, as well as were excellent in their regards the film obtained on Control 6, its P tear strength in the machine direction and uniformity stretchability is obvious. On the other hand, in the case f hi k i h transverse di ti of Controls 1, 3 and 5, measurement of the gauge varia- C l 12 19 ti0 Could h0t be q Sil'lee the films broke Polyethylene terephthalate of an intrinsic viscosity of mg the streiehmg operation 0.65, as measured at C. on a solution in o- Examples 7-16 and Controls 7-11 chlorophenol, was melt-extruded at a temperature of After drying the pellets of polyethylene 2,6-naphtha- 20 280C. through a T die and cooled and solidified on a late of an intrinsic viscosity 0.64 containing 0.125 percooling drum held at 40C. to prepare an undrawn film cent of china clay in a hot air dryer for 3 hours at (thickness 200 microns, width 40 cm). The so obtained 165C., the pellets were melted and extruded at 300C. film was then subjected to biaxial stretching successivethrough aT die, followed by cooling and solidifying on ly under the conditions indicated in Table 3, in the a cooling drum (temperature C.) to obtain an un- 25 machine and the transverse directions in the sequence drawn film (180 microns in thickness and 50 cm in given, using a conventional forward drawing machine width). The so obtained undrawn film was stretched in and a tenter. The relationship between the stretching the machine direction at a draw ratio of either 3.5, 4.0, conditions in the machine and the transverse 4.5 or 4.7 and thereafter stretched in transverse directions, and the stretchability and the properties of direction under the conditions indicated in Table 2 30 the resulting film is shown in Table 3.

TABLE 3 Machine direction Transverse direction stretching stretching Control Tern era- Draw Tern era- Draw Streteh- Gauge variation in the transverse direction No. ture 0.) ratio ture( 0.) ratio ability (percent) 3.5 100 3.0 A 3:10. 85 3.8 100 3.3 A 5:15. 4.0 3.4 A :l:20. 90 4. 5 2. 5 B Unstretched portion coniointly present. 90 4. 5 110 2. 8 B D0. 90 4.5 110 3.0 B D0. 90 4. 5 110 3. 6 B Do. 90 4 5 110 4.0 C

varying the draw ratio. This was followed in all cases by As is apparent from the foregoing results, if in carrya heat treatment for 10 seconds at 210C. The relationing out the successive biaxial stretching of an undrawn ship between the draw ratio the film was stretched in 45 film of polyethylene terephthalate in the machine and the machine direction, the draw ratio the film was transverse directions the film is stretched in the transstretched in the transverse direction, the stretchability verse direction at a draw ratio less than 90 percent of and properties of the film 18 shown in Table 2. that at which it was stretched in the machine direction,

TABLE 2 Tensile strength Youngs Tear strength Gauge Drawing at break modulus* (Elemendorl) variation Draw ratio Draw ratio temperature kg./crn. (kgJcrnJ) (kg/mm.) in the machine transverse transverse transverse Experiment No. direction direction direction C.) MD TD MD TD MD TD direction Example 7 3. 5 3. 1 3, 000 2, 400 67, 000 53, 000 0. 44 0. 58 13 Control 7 3. 5 3. 5 2, 750 2, 690 62, 000 61, 000 0. 61 0. 49 i=2 3. 5 3. 8 2, 580 2, 900 59, 000 64, 000 0. 55 0. 44 5:2 4. 0 2. 6 118 3, 300 1, 800 70, 000 48, 000 0. 30 0. 71 i3 4. 0 3. 3 125 3, 200 2, 000 68, 500 52, 000 0. 38 0. 66 3:3 4. 0 3. 6 130 3, 100 2, 200 68, 000 55, 000 0. 45 0. 55 :!:2 4. 0 3. 8 140 2, 800 2, 500 64, 000 58, 000 0. 45 0. 50 :1:2 4. 5 2. 0 120 3, 800 1, 400 90, 000 38, 000 0. 19 0.82 i8 4. 5 2. 5 125 3, 680 1, 900 86, 000 60, 000 0. 30 0. 68 :1:4 4. 5 2. 8 130 3, 550 2, 380 79, 000 55, 000 0. 35 0. 60 :l:3 4. 5 3. 0 130 3, 450 2, 450 75, 000 57, 0. 40 0. 59 i3 4. 7 2. 2 120 3, 900 1, 500 92, 000 36, 000 0. 20 0. 79 :l:10 4. 7 2. 5 125 3, 850 1, 950 85, 000 52, 000 0. 31 0. 69 i4 4. 7 3. 0 130 3, 500 2, 290 82, 000 58, 000 0. 36 0. 62 :l..3 Example 16 4. 7 3- 3 3, 430 2, 400 ,000 60, 000 0. 30 0. 69 i2 Measured in accordance with Method A.

As apparent from the foregoing results, in the case of either the gauge variation increases or uniformity of the the films of Controls 7, 8 and 9 in which the films were stretching is not achieved. stretched in the transverse direction at draw ratios ex- Ex mple 8 and Controls 20-24 An undrawn film of polyethylene terephthalate (PET) prepared as in Control 12 was biaxially stretched under the conditions indicated in Table 4 successively in the machine and the transverse direction in the sequence given, followed by restretching in the machine direction and thereafter submitting the so stretched film to a heat treatment to prepare a tensilized film. Separately, an undrawn film of polyethylene 2,6-naphthalate (PEN) prepared as in Example 7 was biaxially stretched under the conditions indicated in Table 4 successively in the machine and transverse directions in the sequence given and thereafter submitted to a heat treatment to prepare an oriented film having high tensile strength and Youngs modulus in the machine direction and also possessing excellent dimensional stability. The properties of the several films thus obtained are shown in Table 5.

layer. Hence these films are not desirable for this purpose. On the other hand, the film prepared under such conditions as to reduce the heat shrinkage in the machine direction (Controls 22, 23) has a low tensile strength, Youngs modulus and stress at percent clongation in the machine direction. Consequently. the magnetic tape obtained by coating a magnetic layer on this film is inadequate in respect of its mechanical dimensional stability. Again, in the case of the film which was only biaxially stretched successively in the machine and the transverse directions and thereafter heat treated (Control 24). the tensile strength, Young's modulus and stress at 5 percent elongation were extremely poor.

Examples 19-26 An undrawn film prepared as in Example 7 using TABLE 4.-STRETCHING CONDITIONS Machine direction Transverse direc- Re-stretching in stretching tion stretching machine direction Heat treatment Tempera- Tempera- Temperatempera- Experiment Polyture Draw ture Draw ture Draw ture No. mer 0.) ratio 0.) ratio 0.) ratio C.)

Control 20 PET 90 3.6 110 3. 170 1.4 190 Control 21 PET 90 3. 4 110 3. 6 170 1. 5 190 Control 22- PET 90 3.5 110 .6 200 Control 23 PET 90 3.6 110 .6 205 Control 24 PET 90 4.0 100 200 Example 17 PEN 130 4.5 125 200 Example 18 PEN 135 4. 7 130 200 TABLE 5.FILM PROPERTIES (MACHINE DIRECTION) Tensile Stress at 5% Young's strength Elongation Heat elongation modulus at break I! at break n shrinkage b Experiment No. (kg/cm!) (kg/cm?) (kgJcmJ) (percent) (percent) Control 20. 1, 800 74, 000 3, 600 60 9. 0 Control 21 1, 780 73, 000 3, 400 63 8. Control 2 1, 500 60, 000 3, 000 80 3. 7 Control 23 1, 520 63, 000 2, 980 78 3. 9 Control 24 1, 300 53, 000 2, 950 110 3. 0 Example 17.. 2, 080 79, 000 3, 500 40 3. 1 Example 18. 2,100 80,000 3, 480 38 3. 2

6 Measured in accordance with Method A.

' Heat shrinkage (1 hr., 150 0.).

polyethylene 2,6-naphthalate of an intrinsic viscosity 0.54 was first stretched at a draw ratio of 4.5 at 140C. in the machine direction and then stretched at a draw ratio of 2.8 at 130C. in the transverse direction, after which it was also heat-treated at a temperature and time indicated in Table 6. The relationship between the heat treatment conditions and the properties of the brought about during the step of coating the magnetic resulting film are shown in Table TABLE 6 Tensile strength Elongation Young's Heat Heat Heat at break n at break modulus e shrinkage b treatment treatment (kg./ern. (percent) (kg./cm. (percent) temperatime tare C.) (seo.) Condition of tension MD Tl) M D TD MD TD M1) TD No heat treatment 3, 430 2, 200 30 58 80, 000 48, 000 30 25 170 10 Under tension 3, 480 2, 240 32 60 78,000 51, 000 5. 4 3. 8 170 20 .do- 3, 470 2, 200 33 60 79, 000 50, 500 4. 7 3. 3 210 5 -do 3, 500 2,300 40 78, 800 55, 000 3. 3 2. 0 210 10 .do 3,520 2,330 42 67 78,500 55,800 3, 1 1,0 210 10 Under 3% shrinkage in transverse directio 3, 510 2, 400 41 70 78, 700 55, 400 3, 2 0. 5 240 5 Under tension 3, 530 2, 390 45 70 74, 000 511, 000 2. 2 l, 5 240 5 Under 5% shrinkage in transverse direction- 3, 520 2, 300 44 73 74, 300 ()0 2. 1 (l. 2

' Measured in accordance with Method A. b Shrinkage (1 hr., C.).

Tensile strength at break (kg/cm) 2760 1620 Elongation at break 54 6| Young's modulus (kg/cmfi 73000 45000 Heat shrinkage (200C, 1 min.) 2 3 Measured in accordance with Method B.

Example 29 Polyethylene 2,6-naphthalate (intrinsic viscosity 0.58) cocondensed with 3 mol percent of adipic acid TABLE 7 Tensile Stress at strength at Elongation Young's elongation break at break modulus (kg/cm?) (kgJcmfl) (percent) (kg/0111. Experiment No. Polymer MD TD MD TD MD TD MD TD Example 27... Polyethylene lit-naphthalate- 2,000 1,100 3,100 1,500 40 60 80,000 48,000 Control 25 Polyethylene terephthalate. 1,600 1,000 2, 500 2,300 50 80 62,000 44,000

Measured in accordance with Method B.

Example 28 Polyethylene naphthalate of an intrinsic viscosity of 0.70 obtained in customary manner by the cocondensation of a mixture of 90 mol percent of dimethyl 2,6- naphthalate and mol percent of dimethyl terephthalate was melt-extruded through a T die to obtain an undrawn film. When the so obtained undrawn film was first stretched at a draw ratio of 4.0 at 140C. in the machine direction and then at a draw ratio of 3.0 at 125C. in the transverse direction, the resulting film had the properties shown in Table 8.

TABLE 8 Machine Transverse Direction Direction Stress at 5% elongation (kg/cm"? 1950 1360 Tensile strength at break (kg/cm)* 2540 1500 Elongation at break 34 43 Young's modulus (kg/cm 64000 40000 Heat shrinkage (200C, 1 min) 57 61 Measured in accordance with Method B.

Next, when this film was heat treated for 10 seconds at 190C., the properties of the film were improved as shown in Table 9.

was melt-extruded at 300C. through a T die to obtain an undrawn film (thickness 200 microns). The so obtained undrawn film was first stretched at a draw ratio of 4.0 at 130C. in the machine direction and then at a draw ratio of 3.2 at 128C. in the transverse direction, after which the film was heat treated for 5 seconds at 230C., whereupon a film having the properties shown Measured in accordance with Method A.

Example 30 and Control 36 An undrawn film prepared film polyethylene 2,6- naphthalate of an intrinsic viscosity 0.70 was first stretched at a draw ratio of 4.25 at 130C. in the extrusion direction (machine direction) and thereafter TABLE 9 stretched at a draw ratio of 3.0 at 130C. in the M a chi n e Transverse direction at right angles to the direction of extrusion, Direction Direction after which the stretched film was heat-treated for 10 seconds at 190C. The resistance to hydrolysis of the so Stress at 5 elongation obtained film was compared with that of a polyethylene (kglcm y' 1870 1320 terephthalate film with the results shown in Table l 1.

TABLE 1l.-RESISTANGE 'IO HYDROLYSIS I Test conditions Retension oi tenacity (percent) Stress Tensile Experiment. Thickness Tempera- Pressure at 15% strength Elongation N0. Specimen (mlcrons) turo C.) (km/cm?) elongation at break at break Polyethylene naphthalate filn1 22 160 5 110 85 Example 30; .do 22 186 10 140 135 80 ....(lo 22 210 20 05 60 25 Polyethylene toreplitlialate fi1n1. 25 160 6 110 85 Control 26. .do 25 186 10 so 40 do 2 210 20 Film disintegrated The method of testing the resistance to hydrolysis used in this example was carried out in the following manner. The film in tape form (10 mm wide) was wrapped spirally about a metallic pipe 10 mm in diameter and the ends of the film were fastened. A l-liter autoclave was filled with 100 cc of water and the aforesaid specimen was placed therein in such a manner that the film did not become immersed in the water. The autoclave was then closed and its temperature was raised to the prescribed temperature at a constant rate (3C./min.) Upon reaching the prescribed temperature, the autoclave was purged of the steam and the specimen was taken out and its tensile mechanical properties were measured by the Method A.

Example 31 A 75 micron-thick polyethylene 2,6-naphthalate film was prepared by extruding polyethylene 2,6-naphthalate of an intrinsic viscosity of 0.53 through a flat die, stretching the resulting film in the machine direction at 140C. at a draw ratio of 3.8, then in the transverse direction 3.4 at 130C., and thereafter heat-treating the film for 5 seconds at 230C. The accelerated heat degradation test of the so obtained film was conducted by allowing it to stand in its free state in air of varying elevated temperatures. The results obtained are shown in Table 12. Since it was deemed as a result of this test that the film was fully useable as the material in the class F of thermal classification of electrical insulation, the motorette test was carried out. The results of the motorette test demonstrated that the film had sufficient resistance to the cycles involving heating, vibration and hygroscopicity for 3 days at 240C, days at 220C. and 30 days at 200C. The test shows that film can be used at l75l95C. for years.

film is thin and hence can be satisfactorily put into practical use.

Thus, the film prepared by the invention method possesses particular advantages when used as the base support of the magnetic recording tape. For example, a magnetic recording tape can be made by the formation of a thin layer of magnetic particles on the surface of the film prepared by the invention method. The invention film, as apparent from the hereinbefore given examples, has a small heat shrinkage. Hence it is convenient because the shrinkage of the base film is prevented during the process of forming the thin layer of magnetic particles thereon. The resulting magnetic recording tape has, as hereinbefore noted, the superior physical properties in the machine direction even though the base film is thin. in particular, since the film possesses a high stress at 5 percent elongation and a high Youngs modulus, no distortion of recording occurs even though it is momentarily subjected to a considerably great tensile force. Again, since this film has the hereinbefore noted excellent properties such as to make possible its satisfactory use as the base support of magnetic recording tape even though it is thinner than the conventional films, either the recording time can be made longer with a package of the same size or the package can be made smaller when the invention film is used as the base support in making a magnetic recording tape. Thus, the invention film has great advantages.

Further, the film prepared by the invention method can be put into various decorative or commercial uses by electroplating or vapor deposition of it with metal or by coating it with metallic powders using adhesives. Alternatively, the metal-coated film can be formed into fibers and be used for decorative or electroconductive fibers which are required to have high tensile strength TABLE 12.-PRINCIPAL PROPERTIES AFTER 30 DAYS AT 230 C.

Values of properties Initial after 30 Properties value daysX230 C. Remarks Specific gravity 1. 362 1. 370 Degree of crystallinity, percent... 44 Density method. Tensile strength at break, kg./cm. 2, 100 1, 070 Measured by Method B. Elongation at break, percent 49 Do. Youngs modulus, kg./cm. 50, 700 74, 000 Do. Dielectric strength, kv./mm 20 215 Short time test (ASTM D449). Volume resistivity, KL 1. 5X10 16 1. 2X10 1 min. value at 20C. Dielectric constant"..- 3. 21 2. At C. 60 Hz. Dielectric power factor 6. 5X10 5.4)(10- At 100 C. 60 Hz.

Hot air type constant temperature even used.

When a polyethylene terephthalate film was thermally degraded under the hereinabove indicated conditions, the film became too brittle to measure its mechanical and electrical properties.

As apparent from the foregoing Tables 5 and 6, it is possible according to the present invention to obtain a biaxially oriented film in which the stress at 5 percent elongation, tensile strength at break and Youngs modulus are very high as well as heat shrinkage is low in the machine direction by stretching biaxially an undrawn film of polyethylene 2,6-naphthalate in the machine and the transverse direction in the sequence given. In addition, as shown in Table 2, this biaxially oriented film obtained by the invention method has only a small amount of gauge variation. Hence, the film prepared according to the invention method has a stress at 5 percent elongation, tensile strength at break and Youngs modulus which are high even though the l. in the machine direction at a draw ratio of 3.5-5 to the original length of said undrawn film at a temperature ranging between 10C. above the second order transition temperature of said polyethylene 2,6-naphthalate and C; and thereafter 2. in the transverse direction at a draw ratio 50-90 percent of that at which the film was stretched in the machine direction, at a temperature not higher than the temperature of stretching in the machine direction ranging between 30C. above said second order transition temperature and 160C,

said temperature further satisfying the following relationship wherein T is the drawing temperature (C.) when stretching the film in the transverse direction, X is the draw ratio when stretching the film in the transverse direction, and n is the refractive index in the thickness direction of the film after stretching in the machine direction.

2. The method of claim 1 wherein said undrawn film is stretched at a draw ratio of 4-5 to its original length in the machine direction and thereafter stretched at a for a period of 2 seconds to minutes after stretching l 5 in the transverse direction.

UNITED STATES PATENT OFFICE QERTIFICATE @F CORECTION Patent No. 3,633,060 Dated August 8, 1972 Inventor(S) Takashi Tanabe, Hiroshi Aoiki, Hitoshi Murakami,

ans Fujio Matsumoto It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 1, paragraph 2, line 5, change "30C." to --3C'.--

Signed and sealed this 23rd day of January 1973.

(SEAL) Attest:

EDWARD M..FLETCHER,JR.

ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents 0. OJOSO (10-69) USCOMM-DC 60376'P69 h u 5 GOVERNHENI PRmr'Nc. OFFICE: I969 O-.\66-334 

2. in the transverse direction at a draw ratio 50-90 percent of that at which the film was stretched in the machine direction, at a temperature not higher than the temperature of stretching in the machine direction ranging between 30*C. above said second order transition temperature and 160*C., said temperature further satisfying the following relationship 25X-770n+1200 < or = T < or = 22X-400n+700 wherein T is the drawing temperature (* C.) when stretching the film in the transverse direction, X is the draw ratio when stretching the film in the transverse direction, and n is the refractive index in the thickness direction of the film after stretching in the machine direction.
 2. The method of claim 1 wherein said undrawn film is stretched at a draw ratio of 4-5 to its original length in the machine direction and thereafter stretched at a draw ratio of 60-70 percent of that at which it was stretched in the machine direction.
 2. stretched in the transverse direction at a temperature in the range of 120*-140*C. at a draw ratio of 60-70 percent of that at which the film was stretched in the machine direction.
 3. The method of claim 1 wherein said undrawn film is
 4. The method of claim 1 wherein the film is heat treated at a temperature in the range of 170*-250*C. for a period of 2 seconds to 5 minutes after stretching in the transverse direction. 